When to move chickens to the outside coop, permanently in Colorado

I have 8 Chickens that are all 7 weeks old.
They have been spending days outside in their coop and run, but I am bringing them indoors during the nights.
It is typically between 40-75 during the day (yes, this is life in spring in Colorado) and can be anywhere from 20-40 degrees F at night.
They do great during the day, even on the chillier days they lay in the sun, perch on their swing, and eat and eat and eat. I am not sure if they are ready to spend the nights outside, or if i should continue bringing them inside until it warms up a bit (probably 2-3 more weeks until we have nice night-time weather).

I am pretty sure they are ready to stay outside at night. However, if you still have them under heat at night, you probably want to stop adding heat for a couple of days before putting them out.

You have done the right thing in letting them out in the day time. That will have given them some exposure and will thicken their feathers accordingly. I think that they are more than ready to stay out in the coop.